Commonly in signal transmission systems, a signal to be transmitted is amplified before transmission. In a typical application a power amplifier (e.g. a radio frequency power amplifier) is used for the amplification. In many implementations, the power amplifier may have non-linear characteristics. For example, the power amplifier may be non-linear with regard to envelope and/or phase.
One way to counteract the effect of non-linear characteristics is to apply digital pre-distortion (DPD) to the signal before it is amplified. The digital pre-distortion may typically be adapted to represent an inverse characteristic of the non-linearity to be counteracted such that the combined effect of the digital pre-distortion and the non-linear amplification results in a linear (or close to linear) amplification.
Various digital pre-distortion methods are well known in the art. Typically, a digital pre-distortion arrangement comprises an actuator part in a forward signal processing path. The actuator applies a correction to the signal intended for amplification and transmission. In addition to the forward processing path, a digital pre-distortion arrangement may comprise a feedback path from the output of the amplifier to an adaptor, where the signal output of the amplifier is compared to the non-predistorted signal. The adaptor typically produces correction parameters, which are then fed to the actuator. This enables the actuator to follow variations in the non-linear characteristics of the amplifier and/or to correct discrepancies of the inverse characteristic.
FIG. 1 illustrates an example arrangement for adaptive digital pre-distortion. A similar arrangement is also disclosed in U.S. Pat. No. 5,049,832, which is referred to for further details and examples.
The adaptive digital pre-distortion arrangement of FIG. 1 has a forward processing path 101 and a feedback processing path 102.
A signal intended for transmission is input to a DPD actuator 110 in the forward processing path 101 and the output of the DPD actuator is fed to a power amplifier (PA) 120 for amplification before transmission. The DPD actuator comprises an envelope extractor (ENV) 111, a look-up table (LUT) 112 and a signal combiner (e.g. a mixer) 113. The envelope extractor 111 determines the envelope of the signal intended for transmission and uses the determined envelop value for addressing the look-up table 112. Based on the determined envelope, the look-up table outputs a compensation signal adapted to counteract any non-linearity of the power amplifier 120. The compensation signal is combined with the signal intended for transmission by the signal combiner 113 to produce the output of the DPD actuator 110.
The signal intended for transmission is also input to a DPD adaptation arrangement 130 in the feedback path 102. Also input to the DPD adaptation arrangement 130 is a feedback signal from the output of the power amplifier 120. The DPD adaptation arrangement comprises a delay arrangement 131 and an adaptor 132. The signal intended for transmission is delayed in the delay arrangement 131 (preferably to match the timing of the feedback signal) and compared with the feedback signal in the adaptor 132. The comparison results in a correction signal being output from the DPD adaptation arrangement. The correction signal is used for updating the look-up table 112 as required to improve the linearization properties of the digital pre-distortion.
“Toward a theory of multirate nonlinear systems” by Roberto Lopez-Valcare and Soura Dasgupta, Proc. 2006 Signal Processing Advances for Wireless Communications Workshop (SPAWC'06), Cannes, France, 2006 discloses another example arrangement for digital pre-distortion. The arrangement of this disclosure is without feedback path. In one example, the arrangement uses a poly-phase implementation.
“A memory polynomial predistorter implemented using TMS320C67xx” by L. Ding, H. Qian, N. Chen and G. T. Zhou, Proc. Texas Instruments Developer Conference, Houston, Tex., February 2004 discloses yet another example arrangement for digital pre-distortion. The arrangement of this disclosure comprises a feedback path for adaptation of the pre-distorter.
In applications where two or more signals that are widely separated in frequency are to be amplified simultaneously, the implementation of traditional digital pre-distortion may be cumbersome (or even impossible with the hardware solutions currently available) as will be explained in the following. One example of such an application is when the respective carriers of the two or more signals are in different frequency bands.
Digital pre-distortion is a digital signal process using sampled signals. The sampling rate (signal processing bandwidth) that is necessary to adequately carry out the digital pre-distortion depends on the frequency bandwidth that should be covered by the process. For example, applications with complex signals (i.e. using in-phase I and quadrature Q components) requires a sampling rate that is at least as high as the applicable signal bandwidth according to the Nyquist sampling theorem.
Thus, when two or more signals that are widely separated in frequency are subject of digital pre-distortion, a high sampling rate is required to cover the instantaneous signal bandwidth (IBW). The instantaneous signal bandwidth is defined as the total bandwidth encompassing all the carriers intended for transmission.
In some applications it is also required to cover one, several or all intermodulation (IM) products of the two or more signals to properly implement the digital pre-distortion. For example, to be able to handle third order intermodulation products for carriers contained within 20 MHz, the required signal processing speed (sampling rate) is 3×20=60 MHz, and for fifth order intermodulation products the required signal processing speed is 5×20=100 MHz. When a signal to be transmitted comprises multiple carrier bands separated by a significant frequency span (e.g. 10-100 times the channel bandwidth), covering the intermodulation products of each carrier band and possibly also inter-band intermodulation products of the carrier bands may be required to properly implement the digital pre-distortion. Hence, the sampling rate has to be even higher in such applications to encompass the instantaneous signal bandwidth and the intermodulation products that are to be reduced by the digital pre-distortion process. For example, a signal including simultaneous transmission in Third Generation Partnership Project (3GPP) Band 1 (2110-2170 MHz) and Band 7 (2620-2690) has a maximum frequency span (instantaneous signal bandwidth) of 2690−2110=580 MHz. To handle third order intermodulation products between the two bands, a sampling rate of at least 3×580=1740 MHz would be required in the digital pre-distortion processing.
Hence, a very high sampling rate (in some situations in the same order as radio frequencies) may be required to properly implement the digital pre-distortion. A very high sampling rate is typically not possible to accommodate in currently available hardware implementations (e.g. a field-programmable gate array (FPGA) or an application specific integrated circuit (ASIC)).
One solution would be to process the relevant frequency bands separately and calculate cross terms for pre-distortion of the combined carriers. Such a solution, however, require extra digital processing (which may be costly) and will not be able to handle inter-band intermodulation products.
Therefore, there is a need for methods and arrangements for digital pre-distortion of multiple signals having different carrier frequencies.